iii matter a a aroundroundround u u usss p p pure …ncert.online/pdf/09/sc/cn/ce09sccn02.pdf ·...

Fig. 2.1: Some consumable items

Have you ever noticed the word ‘pure’

written on the packs of these consumables?

For a common person pure means having no

adulteration. But, for a scientist all these things

are actually mixtures of different substances

and hence not pure. For example, milk is

actually a mixture of water, fat, proteins etc.

When a scientist says that something is pure,

it means that all the constituent particles of

that substance are the same in their chemical

nature. A pure substance consists of a single

type of particles. In other words, a substance

is a pure single form of matter.

As we look around, we can see that most

of the matter around us exist as mixtures of

two or more pure components, for example,

sea water, minerals, soil etc. are all mixtures.

2.1 What is a Mixture?

Mixtures are constituted by more than one

kind of pure form of matter. We know that

dissolved sodium chloride can be separated

from water by the physical process of

evaporation. However, sodium chloride is itselfa pure substance and cannot be separated byphysical process into its chemical constituents.Similarly, sugar is a substance which containsonly one kind of pure matter and itscomposition is the same throughout.

Soft drink and soil are not single puresubstances. Whatever the source of apure substance may be, it will always havethe same characteristic properties.

Therefore, we can say that a mixturecontains more than one pure substance.

2.1.1 TYPES OF MIXTURES

Depending upon the nature of the componentsthat form a mixture, we can have differenttypes of mixtures.

Activity ______________ 2.1

• Let us divide the class into groups A,

B, C and D.

• Group A takes a beaker containing

50 mL of water and one spatula full ofcopper sulphate powder. Group B takes50 mL of water and two spatula full ofcopper sulphate powder in a beaker.

• Groups C and D can take different

amounts of copper sulphate andpotassium permanganate or commonsalt (sodium chloride) and mix the givencomponents to form a mixture.

• Report the observations on the

uniformity in colour and texture.

• Groups A and B have obtained a

mixture which has a uniformcomposition throughout. Such

mixtures are called homogeneous

mixtures or solutions. Some other

examples of such mixtures are: (i) saltdissolved in water and (ii) sugar

How do we judge whether milk, ghee, butter,salt, spices, mineral water or juice that webuy from the market are pure?

2

IIIIISSSSS M M M M MATTERATTERATTERATTERATTER A A A A AROUNDROUNDROUNDROUNDROUND U U U U USSSSS P P P P PUREUREUREUREURE?????

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dissolved in water. Compare thecolour of the solutions of the two

groups. Though both the groups have

obtained copper sulphate solution but

the intensity of colour of the solutionsis different. This shows that a

homogeneous mixture can have a

variable composition.

• Groups C and D have obtained

mixtures, which contain physically

distinct parts and have non-uniform

compositions. Such mixtures are called

heterogeneous mixtures. Mixtures ofsodium chloride and iron filings, salt

and sulphur, and oil and water are

examples of heterogeneous mixtures.

Activity ______________ 2.2

• Let us again divide the class into four

groups – A, B, C and D.

• Distribute the following samples to

each group:

− Few crystals of copper sulphate to

group A.

− One spatula full of copper

sulphate to group B.

− Chalk powder or wheat flour to

group C.

− Few drops of milk or ink to

group D.

• Each group should add the given

sample in water and stir properly using

a glass rod. Are the particles in the

mixture visible?

• Direct a beam of light from a torch

through the beaker containing the

mixture and observe from the front.

Was the path of the beam of lightvisible?

• Leave the mixtures undisturbed for a

few minutes (and set up the filtration

apparatus in the meantime). Is themixture stable or do the particles begin

to settle after some time?

• Filter the mixture. Is there any residue

on the filter paper?Discuss the results and form an

opinion.

• Groups A and B have got a solution.

• Group C has got a suspension.

• Group D has got a colloidal solution.

Now, we shall learn about solutions,suspensions and colloidal solutions in thefollowing sections.

uestions1. What is meant by a substance?2. List the points of differences

between homogeneous andheterogeneous mixtures.

2.2 What is a Solution?A solution is a homogeneous mixture of twoor more substances. You come across varioustypes of solutions in your daily life. Lemonade,soda water etc. are all examples of solutions.Usually we think of a solution as a liquid thatcontains either a solid, liquid or a gasdissolved in it. But, we can also have solidsolutions (alloys) and gaseous solutions (air).In a solution there is homogeneity at theparticle level. For example, lemonade tastes thesame throughout. This shows that particles ofsugar or salt are evenly distributed in thesolution.

Q

Fig. 2.2: Filtration

Alloys: Alloys are mixtures of two ormore metals or a metal and a non-metaland cannot be separated into theircomponents by physical methods. Butstill, an alloy is considered as a mixturebecause it shows the properties of itsconstituents and can have variablecomposition. For example, brass is amixture of approximately 30% zinc and70% copper.

IS MATTER AROUND US PURE? 15

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A solution has a solvent and a solute as itscomponents. The component of the solutionthat dissolves the other component in it(usually the component present in largeramount) is called the solvent. The componentof the solution that is dissolved in the solvent(usually present in lesser quantity) is calledthe solute.

Examples:

(i) A solution of sugar in water is a solidin liquid solution. In this solution,sugar is the solute and water is thesolvent.

(ii) A solution of iodine in alcohol knownas ‘tincture of iodine’, has iodine (solid)as the solute and alcohol (liquid) asthe solvent.

(iii) Aerated drinks like soda water etc., aregas in liquid solutions. These containcarbon dioxide (gas) as solute andwater (liquid) as solvent.

(iv) Air is a mixture of gas in gas. Air is ahomogeneous mixture of a number ofgases. Its two main constituents are:oxygen (21%) and nitrogen (78%). Theother gases are present in very smallquantities.

Properties of a solution

• A solution is a homogeneous mixture.• The particles of a solution are smaller

than 1 nm (10-9 metre) in diameter. So,they cannot be seen by naked eyes.

• Because of very small particle size, theydo not scatter a beam of light passingthrough the solution. So, the path oflight is not visible in a solution.

• The solute particles cannot beseparated from the mixture by theprocess of filtration. The solute particlesdo not settle down when left undisturbed,that is, a solution is stable.

2.2.1 CONCENTRATION OF A SOLUTION

In activity 2.2, we observed that groups A and

B obtained different shades of solutions. So,

we understand that in a solution the relative

proportion of the solute and solvent can bevaried. Depending upon the amount of solutepresent in a solution, it can be called a dilute,concentrated or a saturated solution. Diluteand concentrated are comparative terms. Inactivity 2.2, the solution obtained by group Ais dilute as compared to that obtained bygroup B.

Activity ______________ 2.3

• Take approximately 50 mL of watereach in two separate beakers.

• Add salt in one beaker and sugar orbarium chloride in the second beakerwith continuous stirring.

• When no more solute can be dissolved,heat the contents of the beaker to raisethe temperature by about 5°C.

• Start adding the solute again.

Is the amount of salt and sugar or bariumchloride, that can be dissolved in water at agiven temperature, the same?

At any particular temperature, a solutionthat has dissolved as much solute as it iscapable of dissolving, is said to be a saturatedsolution. In other words, when no more solutecan be dissolved in a solution at a giventemperature, it is called a saturated solution.The amount of the solute present in thesaturated solution at this temperature is calledits solubility.

If the amount of solute contained in asolution is less than the saturation level, it iscalled an unsaturated solution.

What would happen if you were to take asaturated solution at a certain temperatureand cool it slowly.

We can infer from the above activity thatdifferent substances in a given solvent havedifferent solubilities at the same temperature.

The concentration of a solution is the amount(mass or volume) of solute present in a givenamount (mass or volume) of solution.

There are various ways of expressing theconcentration of a solution, but here we willlearn only three methods.

(i) Mass by mass percentage of a solution

Mass of solute= ×100

Mass of solution

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(ii) Mass by volume percentage of a solution


Volume of solution

(iii) Volume by volume percentage of asolution

Volume of solute= ×100

Volume of solution

Example 2.1 A solution contains 40 g ofcommon salt in 320 g of water.Calculate the concentration in terms ofmass by mass percentage of thesolution.

Solution:

Mass of solute (salt) = 40 gMass of solvent (water) = 320 g

We know,Mass of solution = Mass of solute +

Mass of solvent= 40 g + 320 g= 360 g

Mass percentage of solution


Massof solution

40

= ×100 =11.1%360

2.2.2 What is a suspension?

Non-homogeneous systems, like thoseobtained by group C in activity 2.2, in whichsolids are dispersed in liquids, are calledsuspensions. A suspension is a heterogeneousmixture in which the solute particles do notdissolve but remain suspended throughoutthe bulk of the medium. Particles of asuspension are visible to the naked eye.

Properties of a Suspension

• Suspension is a heterogeneousmixture.

• The particles of a suspension can be seen

by the naked eye.

• The particles of a suspension scatter a

beam of light passing through it and

make its path visible.

• The solute particles settle down when a

suspension is left undisturbed, that is,

a suspension is unstable. They can be

separated from the mixture by the

process of filtration. When the particles

settle down, the suspension breaks and

it does not scatter light any more.

2.2.3 WHAT IS A COLLOIDAL SOLUTION?

The mixture obtained by group D in activity

2.2 is called a colloid or a colloidal solution.

The particles of a colloid are uniformly spread

throughout the solution. Due to the relatively

smaller size of particles, as compared to that of

a suspension, the mixture appears to be

homogeneous. But actually, a colloidal solution

is a heterogeneous mixture, for example, milk.

Because of the small size of colloidal

particles, we cannot see them with naked eyes.

But, these particles can easily scatter a beam

of visible light as observed in activity 2.2. This

scattering of a beam of light is called the

Tyndall effect after the name of the scientist

who discovered this effect.

Tyndall effect can also be observed when a

fine beam of light enters a room through a small

hole. This happens due to the scattering of light

by the particles of dust and smoke in the air.

Fig. 2.3: (a) Solution of copper sulphate does not show

Tyndall effect, (b) mixture of water and milk

shows Tyndall effect.

(a) (b)

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Q

• They cannot be separated from themixture by the process of filtration. But,a special technique of separation knownas centrifugation (perform activity 2.5),can be used to separate the colloidalparticles.

The components of a colloidal solution are

the dispersed phase and the dispersion

medium. The solute-like component or the

dispersed particles in a colloid form the

dispersed phase, and the component in which

the dispersed phase is suspended is known

as the dispersing medium. Colloids are

classified according to the state (solid, liquid

or gas) of the dispersing medium and the

dispersed phase. A few common examples are

given in Table 2.1. From this table you can

see that they are very common everyday life.

uestions1. Differentiate between homogen-

eous and heterogeneous mixtureswith examples.

2. How are sol, solution andsuspension different from eachother?

3. To make a saturated solution,36 g of sodium chloride is dissolvedin 100 g of water at 293 K.Find its concentration at thistemperature.

Tyndall effect can be observed whensunlight passes through the canopy of a denseforest. In the forest, mist contains tiny dropletsof water, which act as particles of colloiddispersed in air.

Fig. 2.4: The Tyndall effect

Properties of a colloid

• A colloid is a heterogeneous mixture.• The size of particles of a colloid is too

small to be individually seen by nakedeyes.

• Colloids are big enough to scatter abeam of light passing through it andmake its path visible.

• They do not settle down when leftundisturbed, that is, a colloid is quitestable.

Table 2.1: Common examples of colloids

Dispersed Dispersing Type Examplephase Medium

Liquid Gas Aerosol Fog, clouds, mist

Solid Gas Aerosol Smoke, automobile exhaust

Gas Liquid Foam Shaving cream

Liquid Liquid Emulsion Milk, face cream

Solid Liquid Sol Milk of magnesia, mud

Gas Solid Foam Foam, rubber, sponge, pumice

Liquid Solid Gel Jelly, cheese, butter

Solid Solid Solid Sol Coloured gemstone, milky glass

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2.3 Separating the Componentsof a Mixture

We have learnt that most of the naturalsubstances are not chemically pure. Differentmethods of separation are used to getindividual components from a mixture.Separation makes it possible to study anduse the individual components of a mixture.

Heterogeneous mixtures can be separatedinto their respective constituents by simplephysical methods like handpicking, sieving,filtration that we use in our day-to-day life.Sometimes special techniques have to be usedfor the separation of the components of amixture.

2.3.1 HOW CAN WE OBTAIN COLOURED

COMPONENT (DYE) FROM BLUE/BLACK INK?

Activity ______________ 2.4

• Fill half a beaker with water.

• Put a watch glass on the mouth of thebeaker (Fig. 2.5).

• Put few drops of ink on the watch glass.

• Now start heating the beaker. We donot want to heat the ink directly. Youwill see that evaporation is taking placefrom the watch glass.

• Continue heating as the evaporationgoes on and stop heating when you donot see any further change on thewatch glass.

• Observe carefully and record your

observations.

Now answer

• What do you think has got evaporated

from the watch glass?

• Is there a residue on the watch glass?

• What is your interpretation? Is ink a

single substance (pure) or is it a

mixture?

We find that ink is a mixture of a dye in

water. Thus, we can separate the volatile

component (solvent) from its non-volatile

solute by the method of evaporation.

2.3.2 HOW CAN WE SEPARATE CREAM

FROM MILK?

Now-a-days, we get full-cream, toned and

double-toned varieties of milk packed in poly-

packs or tetra packs in the market. These

varieties of milk contain different amounts

of fat.

Activity ______________ 2.5

• Take some full-cream milk in a test

tube.

• Centrifuge it by using a centrifuging

machine for two minutes. If a

centrifuging machine is not available

in the school, you can do this activity

at home by using a milk churner, used

in the kitchen.

• If you have a milk dairy nearby, visit it

and ask (i) how they separate cream

from milk and (ii) how they make

cheese (paneer) from milk.

Now answer

• What do you observe on churning the

milk?

• Explain how the separation of cream

from milk takes place.

Sometimes the solid particles in a liquid

are very small and pass through a filter paper.

For such particles the filtration technique

cannot be used for separation. Such mixturesFig. 2.5: Evaporation

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are separated by centrifugation. The principle

is that the denser particles are forced to the

bottom and the lighter particles stay at the

top when spun rapidly.

Applications

• Used in diagnostic laboratories for

blood and urine tests.

• Used in dairies and home to separate

butter from cream.

• Used in washing machines to squeeze

out water from wet clothes.

2.3.3 HOW CAN WE SEPARATE A MIXTURE

OF TWO IMMISCIBLE LIQUIDS?

Activity ______________ 2.6

• Let us try to separate kerosene oil

from water using a separating funnel.

• Pour the mixture of kerosene oil and

water in a separating funnel (Fig. 2.6).

• Let it stand undisturbed for sometime

so that separate layers of oil and water

are formed.

• Open the stopcock of the separating

funnel and pour out the lower layer of

water carefully.

• Close the stopcock of the separating

funnel as the oil reaches the stop-cock.

Applications

• To separate mixture of oil and water.

• In the extraction of iron from its ore,

the lighter slag is removed from the

top by this method to leave the molten

iron at the bottom in the furnace.

The principle is that immiscible liquids

separate out in layers depending on their

densities.


OF SALT AND CAMPHOR?

We have learnt in chapter 1 that camphor

changes directly from solid to gaseous state

on heating. So, to separate such mixtures that

contain a sublimable volatile component from

a non-sublimable impurity, the sublimation

process is used (Fig. 2.7). Some examples of

solids which sublime are ammonium chloride,

naphthalene and anthracene.

Fig. 2.7: Separation of camphor and salt by

sublimationFig. 2.6: Separation of immiscible liquids

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2.3.5 IS THE DYE IN BLACK INK A SINGLE

COLOUR?

Activity ______________ 2.7

• Take a thin strip of filter paper.• Draw a line on it using a pencil,

approximately 3 cm above the loweredge [Fig. 2.8 (a)].

• Put a small drop of ink (water soluble,that is, from a sketch pen or fountainpen) at the centre of the line. Let it dry.

• Lower the filter paper into a jar/glass/beaker/test tube containing water sothat the drop of ink on the paper is justabove the water level, as shown in Fig.2.8(b) and leave it undisturbed.

• Watch carefully, as the water rises upon the filter paper. Record yourobservations.

This process of separation of componentsof a mixture is known as chromatography.Kroma in Greek means colour. This techniquewas first used for separation of colours, sothis name was given. Chromatography is thetechnique used for separation of those solutesthat dissolve in the same solvent.

With the advancement in technology,newer techniques of chromatography havebeen developed. You will study aboutchromatography in higher classes.

Applications

To separate• colours in a dye• pigments from natural colours• drugs from blood.


OF TWO MISCIBLE LIQUIDS?

Activity ______________ 2.8

• Let us try to separate acetone and waterfrom their mixture.

• Take the mixture in a distillation flask.Fit it with a thermometer.

• Arrange the apparatus as shown inFig. 2.9.

• Heat the mixture slowly keeping a closewatch at the thermometer.

• The acetone vaporises, condenses inthe condenser and can be collectedfrom the condenser outlet.

• Water is left behind in the distillationflask.

Fig. 2.8: Separation of dyes in black ink using

chromatography

Now answer

• What do you observe on the filter paper

as the water rises on it?

• Do you obtain different colours on the

filter paper strip?

• What according to you, can be the

reason for the rise of the coloured spot

on the paper strip?

The ink that we use has water as thesolvent and the dye is soluble in it. As thewater rises on the filter paper it takes alongwith it the dye particles. Usually, a dye is amixture of two or more colours. The colouredcomponent that is more soluble in water, risesfaster and in this way the colours getseparated.

Fig.2.9: Separation of two miscible liquids by

distillation

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SCIENCE22

Now answer

• What do you observe as you start

heating the mixture?

• At what temperature does the

thermometer reading become

constant for some time?

• What is the boiling point of acetone?

• Why do the two components separate?

This method is called distillation. It is used

for the separation of components of a mixture

containing two miscible liquids that boil

without decomposition and have sufficient

difference in their boiling points.

To separate a mixture of two or more

miscible liquids for which the difference in

boiling points is less than 25 K, fractional

distillation process is used, for example, for

the separation of different gases from air,

different factions from petroleum products

etc. The apparatus is similar to that for simple

distillation, except that a fractionating

column is fitted in between the distillation

flask and the condenser.

A simple fractionating column is a tube

packed with glass beads. The beads provide

surface for the vapours to cool and condense

repeatedly, as shown in Fig. 2.10.

2.3.7 HOW CAN WE OBTAIN DIFFERENT

GASES FROM AIR ?

Air is a homogeneous mixture and can beseparated into its components by fractionaldistillation. The flow diagram (Fig. 2.11)shows the steps of the process.

Fig. 2.10: Fractional distillation

Fig. 2.11: Flow diagram shows the process of

obtaining gases from air

If we want oxygen gas from air (Fig. 2.12),we have to separate out all the other gasespresent in the air. The air is compressed byincreasing the pressure and is then cooledby decreasing the temperature to get liquidair. This liquid air is allowed to warm-upslowly in a fractional distillation column,where gases get separated at different heightsdepending upon their boiling points.

Answer the following:

• Arrange the gases present in air in

increasing order of their boiling points.

• Which gas forms the liquid first as the

air is cooled?

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it. To remove these impurities, the process ofcrystallisation is used. Crystallisation is aprocess that separates a pure solid in the formof its crystals from a solution. Crystallisationtechnique is better than simple evaporationtechnique as –

• some solids decompose or some, likesugar, may get charred on heating todryness.

• some impurities may remain dissolvedin the solution even after filtration. Onevaporation these contaminate thesolid.

Applications

• Purification of salt that we get from seawater.

• Separation of crystals of alum (phitkari)from impure samples.

Thus, by choosing one of the abovemethods according to the nature of thecomponents of a mixture, we get a puresubstance. With advancements in technologymany more methods of separation techniqueshave been devised.

In cities, drinking water is supplied fromwater works. A flow diagram of a typical waterworks is shown in Fig. 2.13. From this figurewrite down the processes involved to get thesupply of drinking water to your home fromthe water works and discuss it in your class.

Fig. 2.12: Separation of components of air

2.3.8 HOW CAN WE OBTAIN PURE COPPER

SULPHATE FROM AN IMPURE SAMPLE?

Activity ______________ 2.9

• Take some (approximately 5 g) impure

sample of copper sulphate in a chinadish.

• Dissolve it in minimum amount of

water.

• Filter the impurities out.• Evaporate water from the copper

sulphate solution so as to get a

saturated solution.

• Cover the solution with a filter paperand leave it undisturbed at room

temperature to cool slowly for a day.• You will obtain the crystals of copper

sulphate in the china dish.• This process is called crystallisation.

Now answer

• What do you observe in the china dish?

• Do the crystals look alike?

• How will you separate the crystals from

the liquid in the china dish?

The crystallisation method is used topurify solids. For example, the salt we getfrom sea water can have many impurities in

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uestions1. How will you separate a mixture

containing kerosene and petrol

(difference in their boiling points

is more than 25ºC), which are

miscible with each other?

2. Name the technique to separate

(i) butter from curd,

(ii) salt from sea-water,

(iii) camphor from salt.

3. What type of mixtures are

separated by the technique of

crystallisation?

2.4 Physical and ChemicalChanges

In the previous chapter, we have learnt abouta few physical properties of matter. Theproperties that can be observed and specifiedlike colour, hardness, rigidity, fluidity,density, melting point, boiling point etc. arethe physical properites.

The interconversion of states is a physicalchange because these changes occur withouta change in composition and no change inthe chemical nature of the substance.Although ice, water and water vapour all lookdifferent and display different physicalproperties, they are chemically the same.

Both water and cooking oil are liquid buttheir chemical characteristics are different.

QFig. 2.13: Water purification system in water works

They differ in odour and inflammability. Weknow that oil burns in air whereas waterextinguishes fire. It is this chemical propertyof oil that makes it different from water.Burning is a chemical change. During thisprocess one substance reacts with anotherto undergo a change in chemical composition.Chemical change brings change in thechemical properties of matter and we get newsubstances. A chemical change is also calleda chemical reaction.

During burning of a candle, both physicaland chemical changes take place. Can youdistinguish these?

uestions

1. Classify the following aschemical or physical changes:• cutting of trees,• melting of butter in a pan,• rusting of almirah,• boiling of water to form steam,• passing of electric current,

through water and the waterbreaking down into hydrogenand oxygen gases,

• dissolving common salt inwater,

• making a fruit salad with rawfruits, and

• burning of paper and wood.2. T ry segregating the things

around you as pure substancesor mixtures.

Q

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2.5 What are the Types of PureSubstances?

On the basis of their chemical composition,substances can be classified either aselements or compounds.

2.5.1 ELEMENTS

Robert Boyle was the first scientist to use theterm element in 1661. Antoine LaurentLavoisier (1743-94), a French chemist, wasthe first to establish an experimentally usefuldefinition of an element. He defined anelement as a basic form of matter that cannotbe broken down into simpler substances bychemical reactions.

Elements can be normally divided intometals, non-metals and metalloids.

Metals usually show some or all of thefollowing properties:

• They have a lustre (shine).• They have silvery-grey or golden-yellow

colour.• They conduct heat and electricity.• They are ductile (can be drawn into

wires).• They are malleable (can be hammered

into thin sheets).• They are sonorous (make a ringing

sound when hit).Examples of metals are gold, silver, copper,

iron, sodium, potassium etc. Mercury is theonly metal that is liquid at room temperature.

Non-metals usually show some or all of thefollowing properties:

• They display a variety of colours.• They are poor conductors of heat and

electricity.• They are not lustrous, sonorous or

malleable.Examples of non-metals are hydrogen,

oxygen, iodine, carbon (coal, coke),bromine, chlorine etc. Some elements haveintermediate properties between those ofmetals and non-metals, they arecalled metalloids; examples are boron,silicon, germanium etc.

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• The number of elements known atpresent are more than 100.Ninety-two elements are naturallyoccurring and the rest are man-made.

• Majority of the elements are solid.

• Eleven elements are in gaseousstate at room temperature.

• Two elements are liquid at roomtemperature–mercury andbromine.

• Elements, gallium and cesiumbecome liquid at a temperatureslightly above room temperature

(303 K).

2.5.2 COMPOUNDS

A compound is a substance composed of twoor more elements, chemically combined with

one another in a fixed proportion.What do we get when two or more elements

are combined?

Activity _____________2.10

• Divide the class into two groups. Give5 g of iron filings and 3 g of sulphurpowder in a china dish to both the

groups.

Group I

• Mix and crush iron filings and sulphur

powder.

Group II

• Mix and crush iron filings and sulphur

powder. Heat this mixture strongly tillred hot. Remove from flame and let themixture cool.

Groups I and II

• Check for magnetism in the materialobtained. Bring a magnet near the

material and check if the material isattracted towards the magnet.

• Compare the texture and colour of the

material obtained by the groups.• Add carbon disulphide to one part of

the material obtained. Stir well and

filter.

• Add dilute sulphuric acid or dilute

hydrochloric acid to the other part of

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SCIENCE26

the material obtained.(Note: teacher

supervision is necessary for this

activity).

• Perform all the above steps with both

the elements (iron and sulphur)

separately.

Now answer

• Did the material obtained by the two

groups look the same?

• Which group has obtained a material

with magnetic properties?

• Can we separate the components of the

material obtained?

• On adding dilute sulphuric acid or

dilute hydrochloric acid, did both the

groups obtain a gas? Did the gas in

both the cases smell the same or

different?

The gas obtained by Group I is hydrogen,

it is colourless, odourless and combustible–

it is not advised to do the combustion test for

hydrogen in the class. The gas obtained by

Group II is hydrogen sulphide. It is a colourless

gas with the smell of rotten eggs.

You must have observed that the products

obtained by both the groups show different

properties, though the starting materials were

the same. Group I has carried out the activity

involving a physical change whereas in case

of Group II, a chemical change (a chemical

reaction) has taken place.

• The material obtained by group I is a

mixture of the two substances. The

substances given are the elements– iron

and sulphur.

• The properties of the mixture are the

same as that of its constituents.

• The material obtained by group II is a

compound.

• On heating the two elements strongly we

get a compound, which has totally

different properties compared to the

combining elements.

• The composition of a compound is the

same throughout. We can also observe

that the texture and the colour of the

compound are the same throughout.

Thus, we can summarise the physical

and chemical nature of matter in the

following graphical organiser :

Table 2.2: Mixtures and Compounds

Mixtures Compounds

1. Elements or compounds just mix 1. Elements react to form new compounds.

together to form a mixture and no

new compound is formed.

2. A mixture has a variable composition. 2. The composition of each new substance

is always fixed.

3, A mixture shows the properties of the 3. The new substance has totally different

constituent substances. properties.

4. The constituents can be seperated 4. The constituents can be separated only

fairly easily by physical methods. by chemical or electrochemical

reactions.

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Whatyou havelearnt• A mixture contains more than one substance (element and/

or compound) mixed in any proportion.

• Mixtures can be separated into pure substances usingappropriate separation techniques.

• A solution is a homogeneous mixture of two or more substances.The major component of a solution is called the solvent, andthe minor, the solute.

• The concentration of a solution is the amount of solute presentper unit volume or per unit mass of the solution.

• Materials that are insoluble in a solvent and have particlesthat are visible to naked eyes, form a suspension. A suspensionis a heterogeneous mixture.

• Colloids are heterogeneous mixtures in which the particle sizeis too small to be seen with the naked eye, but is big enough toscatter light. Colloids are useful in industry and daily life. Theparticles are called the dispersed phase and the medium inwhich they are distributed is called the dispersion medium.

• Pure substances can be elements or compounds. An elementis a form of matter that cannot be broken down by chemicalreactions into simpler substances. A compound is a substancecomposed of two or more different types of elements, chemicallycombined in a fixed proportion.

• Properties of a compound are different from its constituentelements, whereas a mixture shows the properties of itsconstituting elements or compounds.

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SCIENCE28

Exercises

1. Which separation techniques will you apply for theseparation of the following?

(a) Sodium chloride from its solution in water.

(b) Ammonium chloride from a mixture containing sodiumchloride and ammonium chloride.

(c) Small pieces of metal in the engine oil of a car.

(d) Different pigments from an extract of flower petals.

(e) Butter from curd.

(f) Oil from water.

(g) Tea leaves from tea.

(h) Iron pins from sand.

(i) Wheat grains from husk.

(j) Fine mud particles suspended in water.

2. Write the steps you would use for making tea. Use the wordssolution, solvent, solute, dissolve, soluble, insoluble, filtrateand residue.

3. Pragya tested the solubility of three different substances atdifferent temperatures and collected the data as given below(results are given in the following table, as grams of substancedissolved in 100 grams of water to form a saturated solution).

(a) What mass of potassium nitrate would be needed toproduce a saturated solution of potassium nitrate in50 grams of water at 313 K?

(b) Pragya makes a saturated solution of potassium chloridein water at 353 K and leaves the solution to cool at roomtemperature. What would she observe as the solutioncools? Explain.

(c) Find the solubility of each salt at 293 K. Which salt hasthe highest solubility at this temperature?

(d) What is the effect of change of temperature on thesolubility of a salt?

Temperature in K

283 293 313 333 353

Solubility

Potassium nitrate 21 32 62 106 167

Sodium chloride 36 36 36 37 37

Potassium chloride 35 35 40 46 54

Ammonium chloride 24 37 41 55 66

Substance Dissolved

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4. Explain the following giving examples.

(a) saturated solution

(b) pure substance

(c) colloid

(d) suspension

5. Classify each of the following as a homogeneous orheterogeneous mixture.

soda water, wood, air, soil, vinegar, filtered tea.

6. How would you confirm that a colourless liquid given to you ispure water?

7. Which of the following materials fall in the category of a “puresubstance”?

(a) Ice

(b) Milk

(c) Iron

(d) Hydrochloric acid

(e) Calcium oxide

(f) Mercury

(g) Brick

(h) Wood

(i) Air.

8. Identify the solutions among the following mixtures.

(a) Soil

(b) Sea water

(c) Air

(d) Coal

(e) Soda water.

9. Which of the following will show “Tyndall effect”?

(a) Salt solution

(b) Milk

(c) Copper sulphate solution

(d) Starch solution.

10. Classify the following into elements, compounds andmixtures.

(a) Sodium

(b) Soil

(c) Sugar solution

(d) Silver

(e) Calcium carbonate

(f) Tin

(g) Silicon

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SCIENCE30

(h) Coal

(i) Air

(j) Soap

(k) Methane

(l) Carbon dioxide

(m) Blood

11. Which of the following are chemical changes?

(a) Growth of a plant

(b) Rusting of iron

(c) Mixing of iron filings and sand

(d) Cooking of food

(e) Digestion of food

(f) Freezing of water

(g) Burning of a candle.

Group ActivityTake an earthen pot (mutka), some pebbles and sand. Design asmall-scale filtration plant that you could use to clean muddywater.

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iii matter a a aroundroundround u u usss p p pure …ncert.online/pdf/09/sc/cn/ce09sccn02.pdf ·...

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